Field of the Invention
The present invention relates to a polylactic resin composition including a polylactic resin as a main component, and a molding of the polylactic resin composition.
Description of the Related Art
In recent years, there have been various environmental issues represented by global warming, and strong concern for depletion of fossil resources. To solve these problems, efforts for building up a recycling-oriented society are currently being made. Resin materials, most of which are produced from fossil resources, are also required to be lowly environmentally-hazardous with low fossil resource consumptions.
In such a social context, intensive activities are being conducted for developing technologies relating to biomass plastics made from plant or microorganism-derived resources, and for studying replacement of resin materials made from fossil resources. Above all, polylactic acids are highly hoped for because they are available at low prices as their high yields from plant-derived materials have been established, and they have excellent machinability and a relatively high melting point. However, the polylactic acids do not have a sufficient impact resistance relative to general-purpose resins, and their Izod impact strength is from about 2 kJ/m2 to 3 kJ/m2. Hence, it is difficult to employ the polylactic acids as components of electric appliances, and it is necessary to improve their impact resistance in order to use them as constructional materials.
Under such circumstances, various attempts are being made to improve the impact resistance of the polylactic acids. Impact strength or impact resistance can be improved by making the materials largely deformable until they finally break under fast deformation, and also by, for example, blending a plasticizer in the materials to increase relaxation. However, this method inevitably reduces elastic modulus and strength, and cannot be employed for constructional materials.
Hence, polymer blending with a different kind of polymeric material is selected as a method for reforming the polylactic acids and improving their impact resistance, and studied in various manners. When the different kind of polymeric material to be blended is in such a combination relationship with the polylactic acid that they dissolve with each other at the molecular level and form a homogeneous phase, the mixture to be obtained has a mechanical property proportional to the composition, which is indicated by a physical property value that is about intermediate between both of the materials and hence relatively easily predictable, which however means that the mixture cannot be a characteristic material in most of the cases. That is, the obtained material has an increased impact resistance, but also has a side effect attributed to the blended resin. In this way, combination of compatible resins often results in a trade-off between the physical property value requested to be improved and the physical property value requested to be maintained. This requires the reforming resin to be blended at a low ratio, making it impossible to obtain a drastic effect in improving impact resistance.
It is commonly by blending an incompatible polymer that it is possible to improve impact resistance without greatly degrading elastic modulus, strength, and heat resistance, and combinations with rubber materials incompatible with the matrix are often employed. A two-phase or multilayered high-order structure formed of such different kinds of polymeric materials can express physical properties that are based on mechanisms unexplainable by the proportional relationship between the mixing ratio and the physical property values, and can outgrow the antinomic dilemma between the plurality of characteristic values. However, as described in Japanese Patent Application Laid-Open (JP-A) No. 2005-264086 (PTL 1), a blend of a polylactic acid and an impact-resistant polystyrene as a reforming resin is lowly improved in impact resistance even when the content of the reforming resin is 50%, and not of a practically useful level.
One of the factors for which the reforming effect is low in the above-described case is considered to be low affinity between the polylactic acid and the reforming resin. Hence, there are proposed a reforming method by copolymerization with a rubber-like segment, and a method of blending this copolymer with a polylactic acid homopolymer. For example, Japanese Patent (JP-B) No. 3350618 (PTL 2) describes that a polylactic acid can be improved in toughness while keeping transparency and strength, by being formed as a block copolymer obtained by bonding a polylactic segment with an aromatic polyester segment and a polyalkylene ether segment. However, this method has a problem that the physical property that should be maintained is degraded, and the effect of improving impact strength is low, which necessitates increasing the ratio of the copolymerization component, resulting in degradation of transparency and significant degradation of bending elastic modulus.
Meanwhile, for example, JP-B No. 5353341 (PTL 3) describes a method of blending a copolymer with a polylactic acid homopolymer, and presents an attempt to improve impact resistance by blending a polylactic acid with a polyester copolymer. However, this method cannot provide an impact resistance that is sufficient from the viewpoint of practical usefulness, and needs improvement.